CON-2017: remove duplicated CSS properties

Pass background-color different than "transparent" to ``clear-linear-gradient`` mixi
GlobalHeaderAB: a typo
Merge branch 'dev' into CON-2017-remove-duplicated-css-properties

Conflicts:
	skins/oasis/css/core/GlobalHeaderAB.scss
Merge pull request #5544 from Wikia/CON-2017-remove-duplicated-css-properties

CON-2017: remove duplicated CSS properties